Pressure-sensitive adhesives (also called self-adhesives) or PSAs are substances that give the support coated therewith an immediate adhesivity (often called “tack”) at room temperature, which allows its instantaneous adhesion to a substrate under the effect of brief slight pressure. PSAs are widely used for the manufacture of self-adhesive labels which are fastened to articles for the purpose of presenting information (such as a barcode, name, price) and/or for decorative, or also medical purposes. PSAs are also used for the manufacture of self-adhesive tapes for various uses. Mention may be made, for example, besides the transparent adhesive tape widely used in daily life, of: the forming and assembling of cardboard packaging; the protection of surfaces for painting works, in construction; holding electrical cables in the transport industry; the bonding of carpets by double-sided adhesive tapes.
With a view to the manufacture of self-adhesive labels and/or tapes, PSAs are often applied by continuous coating processes to the entire surface of a (where appropriate printable) support layer of large dimensions, in an amount (generally expressed in g/m2) and denoted herein below by the term “weight per unit area”. The support layer is constituted of paper or film of polymer material having one or more layers. The adhesive layer that covers the support layer may itself be covered with a protective non-stick layer (often called a “release liner”), for example composed of a siliconized film. The multilayer system obtained is generally packaged by being wound up in the form of large reels of up to 2 m in width and 1 m in diameter, which can be stored and transported. These films on the reels can also be converted into self-adhesive tapes by cutting and packing in rolls of determined width and length. At this stage, the adhesive layer must be sufficiently cured to avoid bleeding problems. Bleeding problems correspond to the adhesive material flowing away from its original place through a lack of mechanical resistance to shear. Shear conditions can be created by temperature, roll tension, pressure and mechanical effects, and bleeding (also named oozing phenomenon) can appear usually at the sides of the rolls, creating issues with handling, storage and also contamination by dust and undesired particles. It is also essential for the manufactured self-adhesive article that the adhesive layer be sufficiently cured to avoid transfer of adhesive material to surfaces with which it is contacted, notably the user's skin. On the other hand, too strong curing removes any adhesive properties.
Self-adhesive tapes and labels are generally produced with adhesives that are reactive or not, in solvent phase. The purpose of the solvents is to facilitate the mixing, dosing, pumping, etc. during the production of these adhesives and their use, but above all to facilitate their coating in a thin layer, which after evaporation will create the self-adhesive surface. The low dry solids content of these adhesives ensures an application rheology (viscosity) adapted to roller sizing and drying. However, the use of solvents is subject to increasingly strict regulation, and solvents are intended to disappear.
More recently some production lines have been operated with adhesives in the aqueous phase. In this case, although the problems inherent to solvents are avoided, the whole problem of drying the coating remains. (Drying must be progressive for reasons of appearance, water has a high enthalpy of evaporation compared with those of the usual solvents, and here again requires heating and powerful ventilation and extraction. Drying must also be total).
In most cases, drying ovens adapted to the production of self-adhesive tapes and labels (in particular high-performance ones) have an effective length of 10 to 100 meters, are heated by pulsed air (80° C. to 180° C.) and are equipped with powerful extraction means in order to complete the drying, and in the case of adhesives in the solvent phase, in order to keep the emitted vapours below their explosion limit. These methods are very energy-intensive, noisy and heavy in term of maintenance. Indeed, the energy necessary to evaporate water from the coated adhesive composition is very high; thus, the process is not satisfying with regards to energy demand, therefore it is dissatisfying from an environmental point of view.
Whether organic solvent or water is used as a media to coat adhesive material, the liquid form is a hinderance to reaching very high coating weights. Above 150 g/m2, even above 70 g/m2, these more or less liquid coatings, before a significant part of evaporation takes place, would be very difficult to keep in place on a moving non-stick support, as would the necessary machine steps to create a finished assembly.
Thus, for high coating weights, it is necessary to perform the coating in more than one step, which becomes not only a technical hurdle but also is not of any industrial efficiency.
Recently, solvent-free reactive technologies with 100% dry solids have been developed, in particular HMPSA (Hot Melt PSA) and UV cure PSA (generally acrylic PSAs, cross-linkable by UV exposure), and even more recently STPU-PSA Hot-Cure, for example a composition which is hot cross-linkable (and in the presence of moisture). In order to obtain satisfactory properties, such a technology uses a moisture level adapted to the quantity of adhesive (in general terms stoichiometric), and a relatively high temperature (50° C. to 180° C., preferably between 80° C. and 160° C. and most often between 100 and 140° C.) in order to obtain the desired conversion rate in a defined period of time, compatible with the size of the ovens and the production line speeds.
Although this kind of adhesive can be usually coated at high coating weight due to the fact that they can solidify very rapidly after the coating process under cooling conditions, there are some difficulties to achieve high performances in term of mechanical and thermal resistance.
Especially when dealing with conventional HMPSAs that are not intended to be cross-linked adhesives, there is typically a lack of mechanical and thermal resistance due to the molecular weight level of molecules comprised in the adhesive materials. To take examples based on thermal resistance, and with very few exceptions that are difficult to be considered as industrially viable, hot melt adhesive bonding solutions cannot withstand temperature exceeding 150° C. for more than few minutes. A vast majority of applications for HMPSAs would not be workable once surrounding temperature reached more than 100° C. Overall, the polymeric materials involved become soft, or very soft to possibly liquid.
Thus the aim of cross-linking is to increase the molecular weight of molecules, typically polymers, in order to enhance mechanical and thermal resistance to the adhesive material, thus to the adhesive bond. Cross-linking can be performed by chemical reactions, in the presence or not of various possible compounds including catalysts, co-catalyst, water, acidic functions, peroxides, peroxide-activated intermediates, etc. Temperature, radiations, and pressure conditions can be used to enhance speed or effect of these reactions.
In particular, the method used for the cross-linking of adhesives, whether Hot-Melt Pressure-Sensitive Adhesive (HMPSA) or not, is UV curing. According to this method which is often used with 100% solid adhesive (no solvent, no water as media to help the coating process), the adhesive composition is coated onto a surface and the coated support is submitted to ultraviolet radiation. For example, for radical polymerization processes, cross-linking can only be performed on the surface up to a thickness of about 100 microns; indeed, the radiation acts superficially and once the composition is cross-linked at the surface, the cross-linked surface prevents the penetration of the radiation and cross-linking of the internal layer of the composition is not possible.
Thus, for high coating weights, it is necessary to perform the cross-linking in several or many successive steps. Such a process includes a succession of coating steps each followed by a UV curing step. Another disadvantage of the UV curing method with radical polymerization processes is that the cross-linking is stopped as soon as the composition is no longer exposed to ultraviolet radiation; thus, cross-linking cannot continue after the product exits from the end of the production line, for example during storage.
Another method for cross-linking HMPSAs can be the electron-beam method. This method requires very expensive systems, thus is not economically viable.
Although it may require a controlled moisture level and a high temperature in order to ensure that the reaction is rapid, the reactive technology based on thermal cross-linking contains no solvent and requires only a very low level of extraction (mainly useful for the satisfactory operation of the regulation system in the oven or chamber). The fact that it is no longer necessary to dry the coating as in the case of solvent- or water-based technologies allows the use of more compact ovens, namely having an oven or stove-type design and no longer of the dryer type. But the very tacky nature of the coatings prohibits all contact with surfaces, even those treated with non-stick materials (Fluorocarbon (Teflon®) or plasma treatment for example). The systems can be horizontal or vertical, but in all cases must be linear.
In some fields of application of PSAs, it is desirable that the adhesive strength of the labels and/or tapes on the article be resistant to temperature variations. For example, it can be desirable that the adhesive strength be maintained when the adhesive joint that provides the fastening is exposed (and also, therefore, the article coated with the label and/or tape) at a temperature capable of varying over a wide range. Mention may be made, by way of example, of stickers for use on certain components of motor vehicles (or other vehicles) located in the vicinity of the engine, or stickers for use on packaging designed to receive a hot liquid during the conditioning thereof, or else stickers for use on articles (such as tires) which are labeled when hot, at the end of production lines. Mention may also be made of the use of self-adhesive tapes for the assembly of parts for which a good thermal resistance is necessary such as in the case, for example, of the interior trim of aircraft or other vehicles.
Document WO 2009/106699 describes a heat-cross-linkable adhesive composition based on a polyurethane comprising two alkoxysilane-type end groups.
Document EP 2 336 208 describes a heat-cross-linkable adhesive composition, said composition being based on a polyether comprising two hydrolysable alkoxysilane-type end groups.
The compositions of both documents result, after coating on a carrier followed by curing, in a pressure-sensitive adhesive that has advantageous adhesive strength and tack properties. Furthermore, the adhesive joint providing the attachment to an article of the self-adhesive support thus obtained retains the required adhesive strength over a wide temperature range.
Those documents do not mention the production of a self-adhesive article which can be used right after exit from the production line. Besides, those documents do not teach how to obtain a self-adhesive article with a coating weight higher than 500 g/m2.
The curing time needed to obtain these advantageous adhesion properties is a particularly important parameter from the point of view of the industrial production of self-adhesive articles. This is because it determines the dimensions of the apparatus wherein the adhesive composition is cured, and also the corresponding residence time of the coated adhesive layer, or else the energy consumption, and therefore the productivity of the process as a whole. In order to be applicable at industrial scales, a process for the manufacture of adhesive articles must present the following characteristics: short residence time in the apparatus for the curing and high curing level at the exit from the production line. Especially, it is expected that the product can be cut and transported right after exit from the production line, without any bleeding problems.
FIG. 1 shows a system according to the state of the art based on vacuum-box suction, allowing the tape to be reversed, with the adhesive-coated side facing the inside of the loop. Obviously, such a system is complex, difficult to control and high-cost.
There is therefore a need for a thermal cross-linking method which is compact, yet still avoids contact between the adhesive material and the different elements of the device, and has a simple and robust design.
One objective of the present invention is also to obtain a Pressure-Sensitive Adhesive with a homogeneous cross-linking using a one step coating process.